Synthesis of 2D Hexagonal Hematite Nanosheets and the Crystal Growth Mechanism

Uncovering the Size-Dependent Thermal Solid Transformation of Akaganéite

Investigating the structural evolution and phase transformation of iron oxides is crucial for gaining a deeper understanding of geological changes on diverse planets and preparing oxide materials suitable for industrial applications. In this study, in-situ heating techniques are employed in conjunction with transmission electron microscopy (TEM) observations and ex-situ characterization to thoroughly analyze the thermal solid-phase transformation of akaganéite 1D nanostructures with varying diameters. These findings offer compelling evidence for a size-dependent morphology evolution in akaganéite 1D nanostructures, which can be attributed to the transformation from akaganéite to maghemite (γ-Fe2O3) and subsequent crystal growth. Specifically, it is observed that akaganéite nanorods with a diameter of ∼50 nm transformed into hollow polycrystalline maghemite nanorods, which demonstrated remarkable stability without arresting crystal growth under continuous heating. In contrast, smaller akaganéite nanoneedles or nanowires with a diameter ranging from 20 to 8 nm displayed a propensity for forming single-crystal nanoneedles or nanowires through phase transformation and densification. By manipulating the size of the precursors, a straightforward method is developed for the synthesis of single-crystal and polycrystalline maghemite nanowires through solid-phase transformation. These significant findings provide new insights into the size-dependent structural evolution and phase transformation of iron oxides at the nanoscale.

Magnetic biocomposite scaffold based on decellularized tendon ECM and MNP-deposited halloysite nanotubes: physicochemical, thermal, rheological, mechanical andin vitrobiological evaluations

The development of new three-dimensional biomaterials with advanced versatile properties is critical to the success of tissue engineering (TE) applications. Here, (a) bioactive decellularized tendon extracellular matrix (dECM) with a sol-gel transition feature at physiological temperature, (b) halloysite nanotubes (HNT) with known mechanical properties and bioactivity, and (c) magnetic nanoparticles (MNP) with superparamagnetic and osteogenic properties were combined to develop a new scaffold that could be used in prospective bone TE applications. Deposition of MNPs on HNTs resulted in magnetic nanostructures without agglomeration of MNPs. A completely cell-free, collagen- and glycosaminoglycan- rich dECM was obtained and characterized. dECM-based scaffolds incorporated with 1%, 2% and 4% MNP-HNT were analysed for their physical, chemical, andin vitrobiological properties. Fourier-transform infrared spectroscopy, x-ray powder diffractometry and vibrating sample magnetometry analyses confirmed the presence of dECM, HNT and MNP in all scaffold types. The capacity to form apatite layer upon incubation in simulated body fluid revealed that dECM-MNP-HNT is a bioactive material. Combining dECM with MNP-HNT improved the thermal stability and compressive strength of the macroporous scaffolds upto 2% MNP-HNT.In vitrocytotoxicity and hemolysis experiments showed that the scaffolds were essentially biocompatible. Human bone marrow mesenchymal stem cells adhered and proliferated well on the macroporous constructs containing 1% and 2% MNP-HNT; and remained metabolically active for at least 21 din vitro. Collectively, the findings support the idea that magnetic nanocomposite dECM scaffolds containing MNP-HNT could be a potential template for TE applications.

Unravelling the atomistic mechanisms underpinning the morphological evolution of Al-alloyed hematite

Hydrothermal synthesis based upon the use of Al3+ as the dopant and/or ethanol as the solvent is effective in promoting the growth of hematite into nanoplates rich in the (001) surface, which is highly active for a broad range of catalytic applications. However, the underpinning mechanism for the flattening of hematite crystals is still poorly comprehended. To close this knowledge gap, in this work, we have attempted intensive computational modelling to construct a binary phase diagram for Fe2O3-Al2O3 under typical hydrothermal conditions, as well as to quantify the surface energy of hematite crystal upon coverage with Al3+ and ethanol molecules. An innovative coupling of density functional theory calculation, cluster expansion and Monte Carlo simulations in analogy to machine learning and prediction was attempted. Upon successful validation by experimental observation, our simulation results suggest an optimum atomic dispersion of Al3+ within hematite in cases when its concentration is below 4 at% otherwise phase separation occurs, and discrete Al2O3 nano-clusters can be preferentially formed. Computations also revealed that the adsorption of ethanol molecules alone can reduce the specific surface energy of the hematite (001) surface from 1.33 to 0.31 J m-2. The segregation of Al3+ on the (001) surface can further reduce the specific surface energy to 0.18 J m-2. Consequently, the (001) surface growth is inhibited, and it becomes dominant after the disappearance of other surfaces upon their continual growth. This work provides atomistic insights into the synergistic effect between the aluminium textural promoter and the ethanol capping agent in determining the morphology of hematite nanoparticles. The established computation approach also applies to other oxide-based catalysts in controlling their surface growth and morphology, which are critical for their catalytic applications.

Facet-dependent dispersion and aggregation of aqueous hematite nanoparticles

Nanoparticle aggregates in solution controls surface reactivity and function. Complete dispersion often requires additive sorbents to impart a net repulsive interaction between particles. Facet engineering of nanocrystals offers an alternative approach to produce monodisperse suspensions simply based on facet-specific interaction with solvent molecules. Here, we measure the dispersion/aggregation of three morphologies of hematite (α-Fe2O3) nanoparticles in varied aqueous solutions using ex situ electron microscopy and in situ small-angle x-ray scattering. We demonstrate a unique tendency of (104) hematite nanoparticles to maintain a monodisperse state across a wide range of solution conditions not observed with (001)- and (116)-dominated particles. Density functional theory calculations reveal an inert, densely hydrogen-bonded first water layer on the (104) facet that favors interparticle dispersion. Results validate the notion that nanoparticle dispersions can be controlled through morphology for specific solvents, which may help in the development of various nanoparticle applications that rely on their interfacial area to be highly accessible in stable suspensions.

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces

Surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood. This limits the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface. Here, using hematite (α-Fe₂O₃) as a model system, we show how oxygen vacancies overwhelm surface cation-dependent hydroxylation behavior. Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy was used to monitor the adsorption of molecular water and its dissociation to form hydroxyl groups in situ on (001), (012), or (104) facet-engineered hematite nanoparticles. Supported by density functional theory calculations of the respective surface energies and oxygen vacancy formation energies, the findings show how oxygen vacancies are more prone to form on higher energy facets and induce surface hydroxylation at extremely low relative humidity values of 5 × 10^-5%. When these vacancies are eliminated, the extent of surface hydroxylation across the facets is as expected from the areal density of exposed iron cations at the surface. These findings help answer fundamental questions about the nature of reducible metal oxide-water interfaces in natural and technological settings and lay the groundwork for rational design of improved oxide-based catalysts.

Structural-controlled formation of nano-particle hematite and their removal performance for heavy metal ions: A review

Hematite is ubiquitous in nature and holds great promise for a wide variety of applications in many frontiers of environmental issues such as heavy metal remediation in environment. Over the past decades, numerous efforts have been made to control and tailor the crystal structures of hematite to improve its adsorption performance for heavy metal ions (HMIs). It is now well established that the adsorption behavior of hematite nanocrystals is strongly affected by their particle sizes, crystal facet contributions, and defective structures. This review examined the size- and facet-dependent hematite, as well as the defective hematite according to their fabrication methods and growth mechanisms. Furthermore, the adsorption performance of various hematite particles for HMIs were introduced and compared to clarify the structure-active relationships of hematite. We also overviewed the advances in charge distribution (CD)-multisite complexation (MUSIC) modeling studies about the HMIs adsorption at the hematite-water interface and the binding parameters. The Present review systematically describes how the formation conditions impact the structural and surface properties of hematite particles, thereby providing new strategies for enhancing the performance of hematite for environmental remediation.

Particle-based hematite crystallization is invariant to initial particle morphology

Many crystallization processes occurring in nature produce highly ordered hierarchical architectures. Their formation cannot be explained using classical models of monomer-by-monomer growth. One of the possible pathways involves crystallization through the attachment of oriented nanocrystals. Thus, it requires detailed understanding of the mechanism of particle dynamics that leads to their precise crystallographic alignment along specific faces. In this study, we discover a particle-morphology–independent oriented attachment mechanism for hematite nanocrystals. Independent of crystal morphology, particles always align along the [001] direction driven by aligning interactions between (001) faces and repulsive interactions between other pairs of hematite faces. These results highlight that strong face specificity along one crystallographic direction can render oriented attachment to be independent of initial particle morphology.

Understanding the mechanism of particle-based crystallization is a formidable problem due to the complexity of macroscopic and interfacial forces driving particle dynamics. The oriented attachment (OA) pathway presents a particularly challenging phenomenon because it occurs only under select conditions and involves a precise crystallographic alignment of particle faces often from distances of several nanometers. Despite the progress made in recent years in understanding the driving forces for particle face selectivity and alignment, questions about the competition between ion-by-ion crystallization, near-surface nucleation, and OA remain. This study examines hydrothermal conditions leading to apparent OA for hematite using three initial particle morphologies with various exposed faces. All three particle types formed single-crystal or twinned one-dimensional (1D) chain-like structures along the [001] direction driven by the attractive interactions between (001) faces and repulsive interactions between other pairs of hematite faces. Moreover, simulations of the potential of mean force for iron species and scanning transmission electron microscopy (S/TEM) imaging confirm that the formation of 1D chains is a result of the attachment of independently nucleated particles and does not follow the near-surface nucleation or ion-by-ion crystallization pathways. These results highlight that strong face specificity along one crystallographic direction can render OA to be independent of initial particle morphology.

Fate of metal-EDTA complexes during ferrihydrite aging: Interaction of metal-EDTA and iron oxides

The widespread presence of ferrihydrite in the environment makes many contaminants including metal-EDTA complexes being adsorbed on it. However, the fate of metal-EDTA complexes during the transformation of ferrihydrite was poorly understood. Understanding the migration and speciation changes of metal-EDTA adsorbed on ferrihydrite during the transformation was helpful to predict its fate in some natural and engineering environments. In this work, the interaction of the two metal-EDTA complexes (Ni(II)-EDTA and Ca(II)-EDTA) and ferrihydrite during the 9-day transformation of ferrihydrite at different pH values was studied. The results showed that part of EDTA complexing metals changed to non-complexed metals during the ferrihydrite transformation, which was due to the fact that metal in the metal-EDTA exchanged with Fe(III) on ferrihydrite. Besides, different speciation of metal ions migrated during the transformation of ferrihydrite. Meanwhile, Fe(III)-EDTA formed in this process, and the exchange of metal in Ca(II)-EDTA with Fe(III) in ferrihydrite was faster than that of Ni(II)-EDTA. Besides, the presence of metal-EDTA affected the transformation rate of ferrihydrite under neutral and alkaline condition, and metal-EDTA accelerated the dissolution of ferrihydrite to form goethite. Therefore, ferrihydrite and metal-EDTA influenced each other during the transformation of ferrihydrite. The results of this work revealed that the process of metal-EDTA dissolving ferrihydrite not only included the dissociation of metal-EDTA, but also involved the migration of metal ions and affected the transformation of ferrihydrite.

Flexible electrocatalysts: interfacial-assembly of iron nanoparticles for nitrate reduction

We report a novel template-assisted epitaxial assembly strategy to assemble carbon-coated iron nanoparticles on a functionalized carbon cloth (CC/Fe@C). This delicate assembled architecture provides a useful guideline for designing flexible iron-based electrocatalysts.

Combined multiplet theory and experiment for the Fe 2p and 3p XPS of FeO and Fe2O3

The Al K alpha, 1486.6 eV, based x-ray photoelectron spectroscopy (XPS) of Fe 2p and Fe 3p for Fe(III) in Fe₂O₃ and Fe(II) in FeO is compared with theoretical predictions based on ab initio wavefunctions that accurately treat the final, core-hole, multiplets. The principal objectives of this comparison are to understand the multiplet structure and to evaluate the use of both the 2p and 3p spectra in determining oxidation states. In order to properly interpret the features of these spectra and to use the XPS to provide atomistic insights as well as atomic composition, it is necessary to understand the origin of the multiplet energies and intensities. The theoretical treatment takes into account the ligand field and spin-orbit splittings, the covalent mixing of ligand and Fe 3d orbitals, and the angular momentum coupling of the open shell electrons. These effects lead to the distribution of XPS intensity into a large number of final, ionic, states that are only partly resolved with energies spread over a wide range of binding energies. For this reason, it is necessary to record the Fe 2p and 3p XPS spectra over a wide energy range, which includes all the multiplets in the theoretical treatment as well as additional shake satellites. We also evaluate the effects of differing assumptions concerning the extrinsic background subtraction, to make sure our experimental spectrum may be fairly compared to the theory. We conclude that the Fe 3p XPS provides an additional means for distinguishing Fe(III) and Fe(II) oxidation states beyond just using the Fe 2p spectrum. In particular, with the use of the Fe 3p XPS, the depth of the material probed is about 1.5 times greater than for the Fe 2p XPS. In addition, a new type of atomic many-body effect that involves excitations into orbitals that have Fe f,ℓ = 3, symmetry has been shown to be important for the Fe 3p XPS.

Facet-Dependent Photodegradation of Methylene Blue by Hematite Nanoplates in Visible Light

The expression of specific crystal facets in different nanostructures is known to play a vital role in determining the sensitivity toward the photodegradation of organics, which can generally be ascribed to differences in surface structure and energy. Herein, we report the synthesis of hematite nanoplates with controlled relative exposure of basal (001) and edge (012) facets, enabling us to establish direct correlation between the surface structure and the photocatalytic degradation efficiency of methylene blue (MB) in the presence of hydrogen peroxide. MB adsorption experiments showed that the capacity on (001) is about three times larger than on (012). Density functional theory calculations suggest the adsorption energy on the (001) surface is 6.28 kcal/mol lower than that on the (012) surface. However, the MB photodegradation rate on the (001) surface is around 14.5 times faster than on the (012) surface. We attribute this to a higher availability of the photoelectron accepting surface Fe³⁺ sites on the (001) facet. This facilitates more efficient iron valence cycling and the heterogeneous photo-Fenton reaction yielding MB-oxidizing hydroxyl radicals at the surface. Our findings help establish a rational basis for the design and optimization of hematite nanostructures as photocatalysts for environmental remediation.

Two-step route to size and shape controlled gibbsite nanoplates and the crystal growth mechanism

Size and shape-controlled synthesis of gibbsite nanoplates via an additive-free two-step route.